1. Field of the Invention
The present invention relates to zoom lens systems suitable for, for example, compact image pickup apparatuses such as digital cameras and image pickup units for cellular phones and personal digital assistants (PDAs).
2. Description of the Related Art
Image pickup apparatuses such as digital cameras and image pickup units for cellular phones and PDAs have an imaging optical system including an image pickup element such as a charge-coupled device (CCD) sensor or a complementary metal oxide semiconductor (CMOS) sensor. A lens system with a relatively long back focus is demanded of the imaging optical system because it also includes various optical members, such as a low-pass filter and a color correction filter, between the image pickup element and the rearmost lens part. On the other hand, a compact, wide-angle lens system is demanded for size reduction of the entire apparatus using an image pickup element having a relatively small effective light-receiving area.
Simple, compact zoom lenses for use in compact image pickup apparatuses are disclosed in, for example, Japanese Patent Laid-Open Nos. 2003-177314 (Patent Document 1), 2004-4765 (Patent Document 2), 2003-177315 (Patent Document 3), 2004-133058 (Patent Document 4), and 2004-318098 (Patent Document 5). These zoom lenses are negative-lead zoom lenses, including a frontmost lens unit with a negative refractive power.
Zoom lenses according to embodiments disclosed in Patent Document 1 include only three lenses and exhibit poor optical performance.
Zoom lenses according to embodiments disclosed in Patent Document 2 include six lenses and exhibit relatively good optical performance. These zoom lenses, however, have a large total lens length and are thus unsatisfactory in terms of size reduction.
Lens systems according to embodiments disclosed in Patent Document 3 have a relatively high zoom ratio, namely, about 2.5×, but are unsatisfactory in terms of size reduction.
Lens systems according to embodiments disclosed in Patent Document 4 are satisfactory in terms of size reduction, but have a relatively low zoom ratio, namely, about 2.0×. In addition, these lens systems are difficult to use directly for image pickup elements of optical apparatuses such as digital cameras because the systems are mainly intended for image pickup elements of cellular phones and PDAs.
Lens systems according to embodiments disclosed in Patent Document 5 have a high zoom ratio, namely, about 3.0× to 5.0×, although these systems include many lenses and are thus unsatisfactory in terms of size reduction.
In general, a larger number of lenses of each lens unit included in a zoom lens increases the length of the lens unit along the optical axis thereof, and a larger amount of movement of each lens unit in zooming and focusing increases the total lens length of the zoom lens.